Field of the Invention
The present invention relates to an image pickup apparatus, and more particularly, to an image pickup apparatus having a partial readout function or a skip readout function, and a photographing apparatus including the same.
Description of the Related Art
At a factory manufacturing line, an image pickup apparatus for inputting images has come to be used in place of visual inspection conducted by a human inspector. Such an image pickup apparatus is also referred to as a machine vision camera, and used for inspecting various types of components or products with a computer or a digital input/output device. In recent years, in order to improve inspection accuracy, an image pickup apparatus having ten million or more pixels has come to be used. Similarly, in a consumer digital camera, the number of pixels is increasing more and more in order to improve captured image quality.
With the increasing needs for a greater number of pixels and higher image quality, in recent years, as an image pickup element to be used for a camera, a complementary metal-oxide semiconductor (CMOS) sensor of a rolling shutter type is widely used. In the rolling shutter type, lines of image pickup elements are sequentially exposed. Thus, exposure time for carrying out exposure by a sensor is also set by a line unit.
When a moving image is captured by such an image pickup apparatus and a signal is read out from all pixels of a pixel array, readout time is longer as the number of pixels is larger. Consequently, in the case of moving image capturing, the number of captured images per second is reduced. Further, a data amount for outputting a captured video to the outside becomes greater, and thus a fame rate decreases. For example, in the machine vision camera, total readout time changes with the number of pixels for imaging, and the frame rate changes with the number of pixels output to the outside of the image pickup apparatus.
In an inspection system using the machine vision camera, shortening of inspection time is simultaneously demanded. Accordingly, the number of readout pixels is reduced by reading out a pixel signal from only an interest area among those in which images are captured by the camera, thereby increasing a frame rate. When the frame rate can be increased, the inspection time of the entire system can be shortened.
In the rolling shutter type, charge processing for photoelectric conversion and readout processing of a photoelectrically converted signal are closely linked with each other. For example, when total readout time is shortened by selecting the interest area, charge time in a partial area of the sensor may change. At this time, exposure time is not uniform over the entire area of the sensor, and thus uneven luminance may occur.
Thus, in Japanese Patent No. 4757013, there is disclosed an example where when charge time varies from line to line in a photographing sequence of one image, a luminance difference generated due to the difference in charge time is corrected by a correction value. According to Japanese Patent No. 4757013, a luminance gap generated between lines due to line skipping can be reduced.
Japanese Patent Application Laid-Open No. 2010-130398 discloses a problem in that image quality is deteriorated when a NULL area and an OB area of an image sensor are skipped during skipping readout. A black level of a captured image is determined based on an image pickup signal of the OB area. Accordingly, when the OB area is skipped, accuracy of the black level may be reduced. Thus, according to Japanese Patent Application Laid-Open No. 2010-130398, image quality deterioration is reduced by reading out the OB area while skipping the NULL area during readout of the image pickup signal.
Under specific photographing conditions, the correction based on the correction value or an offset unit disclosed in Japanese Patent No. 4757013 may be effective. However, a great volume of correction values need to be stored in order to deal with various photographing scenes. When such correction processing is carried out as post-processing after photographing, image quality may drop.
The technology disclosed in Japanese Patent Application Laid-Open No. 2010-130398 is effective for photographing in the case of long exposure of the rolling shutter. However, when a high-speed shutter is used, a difference in exposure time is generated between lines, causing uneven luminance.
In a camera used for inspection, exposure time to be set varies, and there are various skip setting values depending on needs. In addition, in the camera used for inspection, a high-quality and high-definition captured image is required. To maintain optimal image quality, in a photographing sequence of one image, or between frames during moving image capturing, it is desired that all the areas of the image sensor be exposed for the same exposure time.